Terminal device for a bi-directional radio relay link

ABSTRACT

The bandwidth for transmission and reception in a bi-directional radio relay link with two simultaneous broadcasts and receptions is reduced by half. Each terminal device comprises a first broadcaster for broadcasting a first data signal, via a first antenna, in a first used frequency band identical to that in which a first receiver receives a second data signal via a second antenna. A second receiver receives, via the first antenna, a third data signal with a second frequency band and a second broadcaster broadcasts, via a second antenna, a fourth data signal with the second frequency band.

The present invention relates to terminal equipment for a bidirectionalradio link.

This radio link is primarily intended to transmit in two oppositedirections two digital data signals having bit rates from a few kbit/sto a few hundred Mbit/s over a distance from a few kilometers to around100 kilometers. The useful frequency bands for transmitting thesesignals are in the range from approximately 1 GHz to approximately 100GHz. The data signals support multiplexed digital telephone channels ora digital television signal, for example.

In the prior art, each terminal equipment includes a single antenna foremitting and receiving. The antenna is connected to the output of anemitter through a band-pass filter having a first pass-band and acirculator and to the input of a receiver through the circulator andanother band-pass filter having a second pass-band. The emittertransposes a first data signal from an intermediate frequency to a firstemit carrier frequency in a first predetermined useful frequency bandcorresponding to the first pass-band. A second data signal with a secondcarrier frequency and a second useful frequency band corresponding tothe second pass-band is picked up by the antenna and is then transposedto the intermediate frequency in the receiver.

To prevent interference between the two data signals the carrierfrequencies are different and, more particularly, the first and secondfrequency bands respectively for the first and second data signals arejuxtaposed and separated by a guard band.

The bidirectional radio link is generally symmetrical. The two datasignals generally have the same bit rate and the emitter of one terminalequipment is paired with the receiver of the other terminal equipmentand vice-versa.

In another bidirectional radio link, disclosed in French patentapplication 2 744 308, in each terminal equipment the data signals arerespectively emitted and received with crossed polarization by twoantennas in separate frequency bands separated by an empty frequencyband. A pilot frequency serves as a common local frequency for frequencytransposition in the two equipments, according to the two transmissiondirections. The pilot frequency is transmitted as a pure frequency in adownlink direction in order to serve as a central modulation carrierfrequency for the frequency band in an uplink direction.

In all the above prior art, the frequency spectrum occupied by thebidirectional radio link is at least equal to the sum of the twofrequency bands respectively for the transmission directions.

In order to reduce the global useful frequency bandwidth for emittingand receiving in a terminal equipment of a bidirectional radio link,U.S. Pat. No. 5,691,978 proposes terminal equipments for a bidirectionalradio link emitting and receiving simultaneously; each equipment havinga first emitter and a first receiver. The first emitter emits a firstdata signal at a first carrier frequency identical to that at which thefirst receiver receives a second data signal. The terminal equipmentcomprises a single emit and receive antenna, or an emit antenna and areceive antenna.

The object of the invention is to provide terminal equipment for abidirectional radio link emitting four data signals without using apriori four frequency bands and to reduce the useful bandwidth and thecost of the terminal equipment.

Accordingly, a terminal equipment for a bidirectional radio linkemitting and receiving simultaneously, having a first emitter emitting afirst data signal in a first useful frequency band identical to that inwhich a first receiver receives a second data signal, is characterizedin that it comprises:

a second receiver receiving a third data signal in a second usefulfrequency band via a first antenna, a first circulator and a filterhaving the second useful frequency band as pass-band, and said firstemitter emitting said first data signal in the first useful frequencyband via a filter having the first useful frequency band as pass-band,the first circulator and the first antenna, and

a second emitter emitting a fourth data signal in the second usefulfrequency band via a filter having the second useful frequency band aspass-band, a second circulator and a second antenna, said first receiverreceiving the second data signal in the first useful frequency band viathe second antenna, the second circulator and a filter having the firstuseful frequency band as pass-band.

Because the first useful frequency band is common to the first andsecond data signals and the second useful frequency band is common tothe third and fourth data signals, the global useful frequency band forthe link is halved compared to the prior art. This economy in terms ofthe frequency band obtained by the invention enables twice as manysignals to be transmitted in a given useful frequency band than theprior art.

As will emerge in the remainder of the description, the first and seconddata signals emitted and received in the first common frequency band andthe third and fourth data signals emitted and received in the secondcommon frequency band are separated by respective separate emit andreceive antennas and where applicable by two signal cancellers providedat the input of the first and second receivers in the terminalequipment.

Other features and advantages of the present invention will become moreclearly apparent on reading the following description of pluralpreferred embodiments of the invention, which description is given byway of example only and with reference to the corresponding appendeddrawings, in which:

FIG. 1 is a schematic block diagram of a prior art bidirectional radiolink with terminal equipments including a signal cancellers;

FIG. 2 is a diagram showing that the data signals emitted and receivedin a terminal equipment as shown in FIG. 1 have identical frequencybands;

FIGS. 3 and 4 are schematic block diagrams of a emitter and a receiverin a terminal equipment, respectively;

FIG. 5 is a schematic block diagram of a signal canceller connectedbetween an emitter and a receiver in a terminal equipment as shown inFIG. 1;

FIG. 6 is a schematic block diagram of a bidirectional radio link inwhich each terminal equipment of the invention emits two signals indifferent frequency bands and receives two other signals in the same twofrequency bands;

FIG. 7 is a schematic of a vertical mast supporting an antenna emittingin a vertical plane and an antenna receiving in a vertical plane for aterminal equipment of the invention; and

FIG. 8 is a diagram of the two different frequency bands for eachterminal equipment of the invention.

FIG. 1 shows a bidirectional radio link for simultaneous emitting andreceiving of the type described in U.S. Pat. No. 5,691,978, whichcomprises at the respective ends thereof a terminal equipments TA andTB.

Each terminal equipment TA, TB essentially comprises an emitter EA1,EB1, a receiver RA1, RB1, a three-port circulator CA1, CB1, anemit-receive antenna AA1, AB1 and a signal canceller ASA, ASB.

For example, and as shown in FIG. 3, an emitter EA1, EB1 comprises adifferential modulator MD with four phase states for modulating a datasignal Se to be emitted. In the emitter EA1, EA2, the signal ismodulated around an intermediate frequency FIe, then amplified in anamplifier AM and transposed around an emit frequency f1 in a mixer ME towhich is fed a local signal at a frequency fle from a local oscillatorcircuit OSE. Where applicable, the data signal on the carrier f1 isamplified in an output amplifier AS and filtered in a band-pass filterFE before being fed to an input port of the circulator CA1, CB1 andemitted by the antenna AA1, AB1.

As shown in FIG. 2, the signal S1 emitted by the emitter EA1, EB1occupies a useful frequency band BF1 centered on the emit carrierfrequency f1. For example, the carrier frequency f1 is equal to 23 GHzand the width of the useful frequency band BF1 is 28 MHz, i.e. theuseful frequency band runs from 22.986 GHz to 23.014 GHz for a 155Mbit/s data signal that is amplitude-modulated and phase-modulatedaccording to a QAM modulation with 128 states.

Referring to FIG. 4, the receiver RA1, RB1, comprises, for example, amixer MR for transposing the frequency of a data signal S2 at a carrierfrequency f1 that is received via the antenna AA1, AB1 and is appliedfrom an output port of the circulator CA1, CB1, via the signal cancellerASA, ASB, where applicable via a receive band-pass filter FR and whereapplicable via a low-noise amplifier. The mixer MR transposes thereceive frequency f1 into an intermediate frequency FIr=f1±flr, whereflr is the frequency of a local signal produced by a local oscillatorcircuit OSR. The intermediate frequency signal FIr is then processed byan automatic gain control amplifier ACG, a low-pass or band-pass filterFPB and an equalizer EG with error corrector before or after beingdemodulated in a demodulator DE applying four phase state demodulation,for example, yielding a baseband digital signal Sr.

As also shown in FIG. 2, like the emitted signal S1, the signal S2received via the antenna AA1, AB1 and applied to the receiver RA1 viathe signal canceller ASA, ASB has a spectrum that is included in thefrequency band BF1 around the carrier frequency f1.

A signal canceller, such as the signal canceller ASA in the terminalequipment TA, eliminates a portion K1S1 of the emitted signal S1 that isreinjected into the receiver RA1 in the terminal equipment TA and mixedwith the signal S2 received via the circulator CA1.

The signal canceller ASA may comprise an adaptive digital transversalfilter FTA and a subtractor SUA, as shown in FIG. 5 and as known in theart. The transversal filter FTA produces a convolution product of theemitted data signal S1 sampled at the output of the emitter EA1 andcoefficients of the transversal filter FTA representing an impulseresponse of the emit-receive coupling path via the circulator CA1 forreinjecting an emitted signal portion K1S1. The transversal filter FTAestimates a synthesized emitted signal portion K2S1 that is applied withthe opposite phase to the injected emitted signal portion K1S1 in thesubtractor SUA connected between the output port of the circulator CA1and the input of the receiver RA1

The coefficients K1 and K2 are complex coefficients, i.e. thecoefficient K2 is estimated in amplitude and in phase in the transversalfilter FTA. The coefficient K2 is self-adaptive, i.e. may vary overtime, in order for the sum K1S1-K2S1 to converge toward zero at alltimes.

The signal canceller ASA may be entirely analog, or entirely digital, ormay combine an analog portion for partially canceling the signal K1S1and a digital portion for coarsely estimating the coefficients of thetransversal filter FTA or a digital portion for estimating thecoefficients more precisely in order to refine the correction in orderto eliminate the injected emitted signal portion K1S1 combined with thereceived signal S2. Accordingly, the output of the subtractor SUAdelivers a received signal S2 released from the emitted signal portionK1S1. To estimate the coefficient K2 at the output of the transversalfilter FTA, the emitting of the received signal S2 may be interruptedbriefly and periodically in the terminal equipment TB in order for thesignal portion K1S1 only to be applied to the positive input of thesubtractor SUA and an error signal produced by the subtractor SUA andapplied to the transversal filter FTA to be minimized so as to convergetoward zero.

Referring now to FIG. 6, each terminal equipment TAa, TBa comprises afirst emitter EA1, EB1 and a first receiver RA1, RB1 for emitting afirst signal S1, S2 and receiving a second signal S2, S1, these twosignals having a common frequency band BF1, as in the second embodimentshown in FIG. 6 in conjunction with FIG. 2. The terminal equipment TAa,TBa further comprises a second emitter EA2, EB2 and a second receiverRA2, RB2 so that the bidirectional radio link between the terminalequipment TAa and the terminal equipment TBa is duplicated. Thisduplicated bidirectional radio link therefore comprises:

a first bidirectional radio link with simultaneous emitting andreceiving in which the first modulated data signal S1 in the firstfrequency band BF1 and having the carrier frequency f1 is emitted fromthe emitter EA1 to the receiver RB1 via the directional antennas AA1 andAB2, and a third modulated data signal S3 in a second frequency band BF2is emitted by the emitter EB2 and received by the receiver RA2 via theantennas AB2 and AA1, and

a second bidirectional radio link with simultaneous emitting andreceiving in which the emitter EA2 emits a fourth modulated data signalS4 in the second frequency band BF2 and having the carrier frequency f2via the directional antenna AA2 and received by the receiver RB2 via thedirectional antenna AB1, and the second modulated data signal S2 in thefirst frequency band BF1 is emitted by the emitter EB1 via the antennaAB1 and received by the receiver RA1 via the antenna AA2.

As shown in FIG. 8, the two useful frequency bands BF1 and BF2 arejuxtaposed with a guard band of the order of 10 MHz to 1000 MHz betweenthem and are each used to emit and receive two signals S1 and S2, S3 andS4 at the same time in each terminal equipment TAa, TBa via two separateantennas AA1 and AA2, AB1 and AB2.

As shown in FIG. 7, the antennas AA1 and AA2 in each terminal equipmentTAa, TBa are disposed in a plane perpendicular to the parallelpropagation directions of the two signals emitted and the two signalsreceived.

The antennas AA1 and AA2, AB1 and AB2 may be superposed in a plane alonga vertical at the top of a telecommunication mast PY. Alternatively, theantennas AA1 and AA2, AB1 and AB2 at the top of the mast are juxtaposedhorizontally at the same height above the ground.

In order for a portion, such as the portion K1S1, of the emitted signalS1, S4 not to interfere with the processing of the received signal S2,S3 in the receiver of each terminal equipment, such as the terminalequipment TAa, and in order for the antennas to be relatively decoupled,the distance d between the antennas AA1 and AA2 becomes smaller as therespective frequency f1, f2 of these signals become higher. For afrequency f1, f2 greater than 1 GHz, the distance d is a few tens ofcentimeters. Typically, for an antenna diameter of 15 cm, the distance dis approximately 60 cm, i.e. in the range from a few wavelengths to afew tens of wavelengths. For frequencies less than 1 GHz the distance dis more than one meter. In all cases, care is taken that the interferingsignal radiated laterally by the first antenna AA1 and picked up by thesecond antenna AA2 is such that the ratio of the useful signal to theinterfering signal is sufficient, according to the transmissionmodulation and the transmission quality that are required.

Where appropriate, if the antennas AA1 and AA2, AB1 and AB2 are closetogether and/or not highly directional, each terminal equipment TAa, TBapreferably comprises a signal canceller ASA1 and ASA2, ASBL and ASB2 asshown in dashed line in FIG. 6. The signal canceller ASA1, ASBL isanalogous to that described with reference to FIG. 5 and cancels aportion of the signal S1, S2 emitted by the antenna AA1, AB1 and pickedup by the antenna AA2, AB2 and the signal canceller ASA2, ASB2 isanalogous to that described with reference to FIG. 5 and cancels aportion of the signal S4, S3 emitted by the antenna AA1, AB1 and pickedup by the antenna AA2, AB2.

In each terminal equipment TAa, TBa there are therefore twoemitter-receiver-antenna arrangements analogous to theemitter-receiver-antenna arrangement in the first embodiment of aterminal equipment, shown in FIG. 1, but additionally with band-passfilters. For example, in each of the arrangements of the terminalequipment TAa relating to the two bidirectional radio links S1-S3 andS4-S2, the emitter EA1, EA2 emits the modulated data signal S1, S4 via aband-pass filter FA1, FA4, a three-port circulator CA1, CA2 and thedirectional antenna AA1, AA2, and the receiver RA2, RA1 receives thedata signal S3, S2 from the directional antenna AA1, AA2 via thecirculator CA1, CA2 and a band-pass filter FA3, FA2. The band-passfilters FA1 and FA2 and the band-pass filters FB1 and FB2 at the inputof the receiver RB1 and at the output of the emitter EB1 in the terminalequipment TBa have pass-bands identical to the first frequency band BF1of the first and second data signals S1 and S2. The band-pass filtersFA3 and FA4 and the band-pass filters FB3 and FB4 at the output of theemitter EB2 and at the input of the receiver RB4 in the terminalequipment TBa have pass-bands identical to the second useful frequencyband BF2 of the third and fourth data signals S3 and S4. In each of thetwo emitter-receiver arrangements in the terminal equipment TAa, the twoband-pass filters FA1 and FA3, FA4 and FA2 and the circulator CA1, CA2constitute a duplexer.

Accordingly, the emitter EA1, EA2, the receiver RA2, RA1 and theduplexer FA1-CA1-FA3, FA4-CA2-FA2 with the antenna AA1, AA2 togetherconstitute a standard off-the-shelf terminal equipment, whichconsiderably reduces the cost of the FIG. 8 bidirectional radio link,although the latter has a spectrum reduced to two frequency bands BF1and BF2 for transmitting four data signals S1, S2, S3 and S4, instead offour frequency bands as in the prior art.

1. A terminal equipment for a bidirectional radio link, said terminalequipment being adapted to emit and receive simultaneously andincluding: a first emitter adapted to emit a first data signal in afirst useful frequency band and a first receiver adapted to receive asecond data signal in said first useful frequency band, a secondreceiver adapted to receive a third data signal in a second usefulfrequency band via (a) a first antenna, (b) first circulator, and (c)filter having a passband for passing said second useful frequency band,said first emitter being adapted to emit said first data signal in saidfirst useful frequency band via (a) a filter having a pass-band forpassing said first useful frequency band, (b) said first circulator, and(c) said first antenna, and a second emitter adapted to emit a fourthdata signal in said second useful frequency band via (a) a filter havinga pass-band for passing said second useful frequency band, (b) a secondcirculator, and (c) a second antenna, said first receiver being adaptedto receive the second data signal in said first useful frequency bandvia (a) said second antenna, (b) said second circulator, and (c) afilter having a pass-band for passing said first useful frequency band.